How to install an Inductive Proximity Sensor for Auto Bed Leveling

by quillford Nov 11, 2014
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​I have a couple of questions to be cleared about wiring. First of all, as I understood from @mickeypop 's sayings in here:
http://www.instructables.com/id/Enable-Auto-Leveling-for-your-3D-Printer-Marlin-Fi/?ALLSTEPS , S pin of the RAMPS is "Signal in" pin and it needs to take in 5V voltage. PNP and NPN proximity sensors are different basically as the Black wire could be used as "sinking" or "sourcing" the current as mentioned here:

If the S pin is "sinking" in RAMPS how can we use the NPN proximity sensors as they are used for sourcing pins? Are they different in wiring or what? And is there any difference in voltage dividing of these types? Please could someone clarify this mess in my head?

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Can someone direct me to where he found that plate? Does anyone have any experience with that plate? I wonder if this could fit my old makerfarm i3.

What plate are you referring to?

I get a 3V reading across the 10kohm resistor when the sensor isn't tripped, but get the expected 0V when I disconnect the Zmin wire from the board.
Anyone know how to fix this?


I can do this with a much cheaper sensor that is more accurate than the inductive sensor used here. I use an IR reflective sensor that is accurate to 0.01 mm for a range of up to 10 cm away or as close as 1cm. No need to get too close to your table. Let me know if anyone wants such a sensor.

I'm bidding item on ebay now. Is it possible to share the configuration ?

You can see my response to ShyRider below where I describe the sensor configuration. Its similar to connecting an analog Sharp sensor but it has a pot on it to adjust an offset distance. This will be useful for your 3D printer depending on the reflectivity of the bed surface and distance. Let me know if you have any questions.

I am curious and I am interested and I can try this on my i3. How does it mount and what changes need to be made to firmware?

The sensor is a small PCB with two screw holes, and a connector for 5V, Gnd, and analog out. There is an adjustment pot on the back to set the fixed offset range (like 1 to 8cm), hence no need to adjust the height physically. Depending on the height of your printer bed you can adjust the pot until you get a linear range reading between 0 and 5V. You can set your firmware to trigger on any analog voltage you like. This sensor was designed for a large format printer to detect paper jamming. I can sell you one cheap on eBay to get you started.

Point me at it and I will probably ask you for detailed step by step instructions if you have them.

Sure, no problem. Here is the eBay link:


You can see sample code and a short video operating the sensor.

Thanks. I will grab 2 or 3 of these of for no other reason than to put them on my robots. Hooking up to my 3D printer ideally would mean replacing the Z endstop. If you put together an article here or even an Instructible I am sure we would all appreciate it. For example, can we follow the same procedure here to plug power into the RAMPS directly and regulate the voltage to 5V?

"These measurements don't need to be super accurate, but they should be in the right ball park."
Please: A specific margin of error would be more reassuring.

How thick should the aluminum bed be?

I use 3mm thick aluminum on top of a heated bed.

Wow. With 3mm thick aluminum, about how close to the bed does your sensor need to come to trigger it? My concern is that if the bed is grossly tilted, the sensor might bump into the printed parts as it prints.

The sensor is close enough to the hotend (at least on the printer I have it on) that it isn't a concern. The sensor I use triggers at 3mm. 3mm is sufficient, but if you still have concerns, there are sensors that will trigger at a further distance.

Hi is your bed PCB ou alluminium. I'm asking because in the video, Tom says that it doesn't work with PCV heated bed, but if you did, I'll try!!


I do not use a mk3 heated bed on that printer. On the printer I still use ABL on, I have a regular mk2 with an aluminum plate on top. It does sort of work with just a PCB but isn't quite as reliable.